1. Field
The invention is in the field of computer communication systems, and particularly such systems known as local area networks in which a number of computers and peripheral equipment therefor are interconnected.
2. State of the Art
The computer industry has recently been working on systems of interconnected computers for which the name "local area networks" has been used. A local area network is a system of interconnected computers and peripheral equipment, such as printers, readers, etc., wherein each shares the resources of all of the others. The net result is a system whose power is greater than the sum of the individual parts, and whose cost per station is very low compared with other methods of achieving similar power. The dramatic decrease in per-station cost is the result of sharing expensive hardware, such as disk drives and printers, among many users, while the increase in power comes from coordination of multiprocessing and from multiple access to large, shared data bases.
Although such a system should result in reduced costs, this is not so if the system is designed for high speed transfer of data, since the interfacing components are expensive and can end up costing much more than the interconnected microcomputers and peripherals. One reason for this excessive cost is because the high speed transfer is faster than the computers themselves can assemble and send or receive information, so that, rather than being merely a simple, line connection, the interfacing equipment must either assemble the information to be sent from the computer and then implement the high speed transmission, or it must receive the high speed transmission and then break it down and feed it to the computer at a speed the computer can accept. Further, many of the components needed for certain of these high-speed communication systems are not yet commercially available. Thus, there is a need for a standardized, relatively low-speed communication system for use in local area networks in which the interfacing components merely transmit information that can be assembled in the computers themselves. Systems of this type have been suggested previously, utilizing a standard information packet format whereby data from any unit connected to the local area network can be transmitted to any other unit in such packet format, the communication being done at a standard speed.
Computers and peripheral equipment therefor do not all have the same speed capability. Some of the low-cost computers currently available, as well as some of the peripheral equipment, e.g. printers, operate relatively slowly compared to other currently available computers and peripheral equipment. Thus, while relatively low-speed is desired in order to reduce expense of the overall system, there is always the question of what low-speed should be regarded as standard. With present technology, speed of data exchange above 100 kHz is difficult to reach with low-cost components. Speeds above 1 MHz are difficult to reach even with exotic, expensive components. Further, line problems are encountered when speeds above 1 MHz are used which reduces the distance over which successful communication can be achieved. In addition, such high speeds require use of expensive line-driving equipment. Even at lower speeds, line problems may be encountered where distances are not sufficiently short. Thus speeds should be kept at a minimum, so that low-cost, large systems become possible. In all instances, speed should be no faster than the slowest computers or peripheral equipment that could be connected in the system. However, systems that operate too slowly for a given application are of no use. Certain applications, such as the transmission of pixel graphics from a computer or imaging system, require massive amounts of data interchange, and very large installations having dozens or hundreds of interconnected computers will quickly saturate a system with too low a speed. Accordingly, an ideal network speed must be achieved by compromise. In cost-sensitive applications, the network speed is kept as low as possible considering the job that must be done.